HN has been receiving an influx of Reddit users for quite some time now. I do feel HN's quality has suffered as a result (not due to people coming from Reddit in particular, just the classic problem of quality reduction as communities grow)
The SNR in Reddit dropped when Digg destroyed what little community they had left. I hope HN dodge that particular bullet. I'm a temporary (UK-based) Reddit refugee but I'll be back on Reddit in a few hours. This is my methadone, Reddit is my smack.